1. Field of the Invention
The present invention relates to digital imaging techniques and to digital image capture techniques. More particularly, the present invention relates to electronic density filters and more particularly to independent color channel graduated density filter techniques in digital cameras.
2. The Prior Art
In landscape photography, the dynamic range in the scene often exceeds the capability of image sensors in digital cameras. This is often because the upper portion of the frame includes the sky, which is substantially brighter than the remainder of the subject matter in the frame. This problem also exists with print film.
It is very common therefore to use an optical neutral density filter to compress the dynamic range of a scene to be within the dynamic range of the image sensor. As illustrated in FIG. 1, the density of the filter is graduated and increases from the bottom of the image to the top of the image.
The optical neutral density filter is placed in front of the lens, and is oriented such that the portion of the filter having the highest density is located at the top of the frame so that the image is darkened relative to the bottom. While the density change from the top of the image to the bottom of the image can be linear as shown at reference numeral 10, it is most often non-linear as shown at reference numeral 12.
It is known that an optical neutral density filter can be electronically simulated by placing a variable charge sharing capacitance in parallel with individual pixel sensors in an image sensor or by using a variable capacitance to share the sensed charge of one or more pixel sensor photodiodes during readout. An example of such an electronic neutral density filter for an image sensor is disclosed in U.S. Pat. No. 7,635,833 to Mansoorian. In operation, each photodiode is reset by turning on the appropriate transfer gates, such as 310, at the same time as the reset transistor 325. The photodiode is then allowed to integrate charge.
Because the accumulated photocharge in each pixel sensor is shared between the photodiode and the added capacitor, the pixel sensor operates as though the photodiode received fewer illumination photons by an amount proportional to the value of the capacitor. As the capacitance increases, the size of the photodiode effectively increases, and the sensitivity of the photodiode to incoming illumination decreases.
It is also known to provide an electronic neutral density filter by introducing a controlled delay between the reset signal and the read signal of a digital imaging array. Such a configuration is disclosed in U.S. Pat. No. 8,780,241 to Johnson.